Project Summary Despite great efforts in HIV-1 vaccine development, a vaccine is still lacking that can elicit broadly neutralizing antibodies (bNAb) sufficient enough to block HIV infection. Due to the success of passive infusion of bNAbs in SHIV challenged rhesus macaques to block systemic infection, a new trial was launched this past fall where at risk individuals received an IV infusion of the bNAb, VRC01. However, there are still many questions as to how these specialized antibodies can provide sterilizing immunity. In a recent study by the Barouch lab, it was shown that rhesus macaques that got a -1 day IV infusion of the broadly neutralizing antibody PGT121, prior to intravaginal challenge with SHIV-SF162P3, had distal site virus accumulation 1-3 days after challenge. In addition, they found a higher ratio of viral DNA to viral RNA, which implies that there was active replication at these distal sites despite having been given a broadly neutralizing antibody 1 day before. This shows that what was once thought of as sterilizing immunity does not protect from early infection in the FRT and at distal sites, despite clearing the virus at 10 days. This model provides an excellent opportunity to understand how antibodies clear the virus following acute infection in the FRT and at distal sites. In preliminary data using cy5- labeled VRC01 IV injected into rhesus macaques I have found that it takes antibodies about 1 week to anatomically distribute to achieve steady state levels in the tissue, whereas plasma levels peak much earlier. Utilizing this newly developed labeled antibody platform, I propose in Aim 1 to label PGT121 with cy5 and compare -7 day IV infusion (steady state) and -1 day IV infusion (non-steady state) prior to intravaginal challenge with SHIVSF162P3 in the rhesus macaque and measure female reproductive tract (FRT) and distal site accumulation of virus at 48 hours and 1 week after challenge. In Aim 2, I will measure the antibody levels at these distal tissues as well as in the FRT and compare this with virus accumulation. In the second part of this aim, we will look at the transcriptomic profile of tissues that have virus accumulation compared to those that do not, with and with antibody present in an effort to identify a potential path of virus dissemination. Finally, in Aim 3 I will utilize a novel platform from the Hope lab known as the LICh reporter system, which allows identification of early transmitted foci of infection. I will co-challenge rhesus macaques with SIV pseudotyped LICh and SHIVSF162P3 and in order to elucidate the role of steady state antibody distribution has on the number of foci of infection as well as the extent to which the virus disseminates throughout the entire FRT. In the second part of Aim 3, we will narrow in on these foci of infection and characterize potential immune clearance mechanisms, such as ADCC, identified by the cy5-labeled PGT121. The highlighted aims outlined in this application will provide crucial insight into the role steady state antibody tissue distribution has on the sterilizing protection afforded by bNAbs.